Seeing and Perception. External features of the Eye

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1 Seeing and Perception Deceives the Eye This is Madness D R Campbell School of Computing University of Paisley 1 External features of the Eye The circular opening of the iris muscles forms the pupil, which controls the amount of light entering the eye. Pupil Sclera Iris The pupil opens (dilates) automatically in dim lighting and contracts in bright, like the aperture control of an automatic camera. D R Campbell School of Computing University of Paisley 2 1

2 The Anatomy of the Eye Retina D R Campbell School of Computing University of Paisley 3 Geometric Optics of the Eye The cornea and the fluid behind it bring light to a rough focus on the light sensitive retina. Small muscles tension or relax the flexible lens, changing its shape to make the fine focussing adjustments (accommodation) needed for a sharp image. Pupil Path of Light rays Retina Lens Iris Cornea Light rays Blocked by iris D R Campbell School of Computing University of Paisley 4 2

3 The Structure of the Retina The light sensitive rod, and colour sensitive cone nerve cells lie closely packed together on a pigmented non-reflective backing tissue. The surface layer of the retina carries the blood vessels and the nerve connections to the brain. Layer of Blood Vessels and Nerves Light passed through pupil Retina Surface Cone Cell Backing Tissue Light Sensitive Ends Rod Cell D R Campbell School of Computing University of Paisley 5 Why flash can give red eye Light filters through the upper layers before reaching the light sensitive ends of the rod and cone cells. It is as if the film had been put in the camera with the sensitive side away from the light. Reflected light from the blood vessels in the surface layer is responsible for the red-eye effect seen in some flash photographs. This can be retouched using image processing software. D R Campbell School of Computing University of Paisley 6 3

4 The Fovea and its surrounds The area of greatest visual acuity, the fovea, lies directly behind the pupil in the centre of a small region where the only light sensitive cells are densely packed colour sensitive cone-shaped cells. Around this area the rod and cone cells intermix, with fewer cone-shaped cells toward the periphery. The outer areas have only rod cells, which are more sensitive than cones to low light levels but not sensitive to colour. No light-sensitive cells exist in the blind spot where the optic nerve enters the eyeball. D R Campbell School of Computing University of Paisley 7 Viewing the Retina through the Iris Periphery: Rod cells only Blind spot: Blood vessels & Optic Nerve entry Rod and Cone cells Fovea: Cone cells only Blood vessels Retinal Image courtesy of: Currie & Quirk Opticians Ltd., Glasgow D R Campbell School of Computing University of Paisley 8 4

5 The Visual Field The retinal structure creates a visual field with a small central area capable of high resolution colour vision, surrounded by a less sharp peripheral area. In low illumination peripheral vision may detect objects invisible to central vision. Our eyes are constantly in motion leaving us unaware that our visual field has a central zone of sharpness surrounded by an area of increasing fuzziness. Our eyes also tend to be attracted to the sharply focussed areas of a photographic image. D R Campbell School of Computing University of Paisley 9 Field of Vision of Right Eye Area of Stereoscopic Vision Area of Monoscopic Vision Seen by Rods and some Cones Seen by Cones and some Rods Seen by Cones only Blind Spot This peripheral area seen only by Rod Cells D R Campbell School of Computing University of Paisley 10 5

6 Visual Field: Implications for Displays Peripheral vision is more sensitive to display flicker. So wide field of view displays require high refresh rates. Low illumination displays may look less sharp than higher ones because central vision is not being sufficiently stimulated. Soft focus applied to surrounding areas can concentrate the observers attention on central sharp focussed features. Area of interest displays use eye tracking to exploit features of the visual field and concentrate visual detail only where it can be seen. D R Campbell School of Computing University of Paisley 11 Human Visual Frequency Response With normal vision, each different frequency of visible light causes our eye to see a different colour and we can distinguish about 5 million mixtures. In air, the longest wavelength we can detect visually is about 700 nm (deep red) and the shortest wavelength is about 400 nm (violet). Our best resolution is when using the red/green sensitive cells packed into the fovea and light at around 550 nm (greenísh yellow), which is also the brightest colour in sunlight at the earth's surface. D R Campbell School of Computing University of Paisley 12 6

7 Sensitivity of the Colour Receptors in the Eye Relative Sensitivity Green receptors 30% Blue receptors 5% Red receptors 65% Wavelength of light nm D R Campbell School of Computing University of Paisley 13 Red-Green Colour Vision Defficiency Test About 8% of the male population has defficient colour vision. It is very rare in females (0.5%). Oh say can you see? Not if you re Low on Red D R Campbell School of Computing University of Paisley 14 7

8 Display Screens and Colour Blindness From D R Campbell School of Computing University of Paisley 15 Implication of low resolution of blue Pure blue is often avoided for fine lines and text, but makes a good background. Blue background with White text Pure blue is often avoided for fine lines and text, but makes a good background. Neutral background with Blue text D R Campbell School of Computing University of Paisley 16 8

9 Colour difference sensitivity Difference in blue only ( = 175) Difference in red only ( = 175) D R Campbell School of Computing University of Paisley 17 Colour edge sensitivity Difference in blue only ( = 175) D R Campbell School of Computing University of Paisley 18 9

10 Foci of colours The lens of the eye, like any simple lens, is not colour corrected and brings different colours to a focus at different distances. Red Segment of Retina Green Blue Lens of Eye Partially focussed light rays Cornea Light D R Campbell School of Computing University of Paisley 19 Colour Accommodation This means that the eye has to accommodate (change focus) to see each colour sharply when groups of pure colours are displayed. Segment of Retina Lens of Eye Partially focussed light rays Cornea Light D R Campbell School of Computing University of Paisley 20 10

11 Colour Pairs requiring refocusing Purple Blue Yellow Red D R Campbell School of Computing University of Paisley 21 Which is background, red or blue? D R Campbell School of Computing University of Paisley 22 11

12 Volkswagen Instrument Panel (Day) Fuel Temp 0 ¼ ½ ¾ 1 Rev/min D R Campbell School of Computing University of Paisley 23 Volkswagen Instrument Panel (Night) Fuel Temp 0 ¼ ½ ¾ 1 Rev/min D R Campbell School of Computing University of Paisley 24 12

13 Here s looking at you kid The perception of depth in an image depends on several factors. At distances less than about 50m binocular (stereo) vision is an important, but not over-riding cue. Many other learned associations are incorporated into an overall judgement of the depth ascribed to objects in a scene. D R Campbell School of Computing University of Paisley 25 Binocular Triangulation Having two eyes, and a nervous system that can fuse the two images and estimate the convergence angle, helps us judge distance by triangulation. Left eye 63 mm A Object Right eye d2 d1 63 mm B Distance: d1 > d2 ; Convergence: Angle B > Angle A; D R Campbell School of Computing University of Paisley 26 13

14 Binocular Disparity In addition to triangulation, due to their separation (adult average about 63 mm) each eye forms a slightly different image at the retina. This is known as binocular disparity. Left eye image Right eye image D R Campbell School of Computing University of Paisley 27 Disparity revealed The disparity becomes more obvious when an image processing package is used to subtract the two images. D R Campbell School of Computing University of Paisley 28 14

15 These are ants, we re still on the ground! At distances above 50 m triangulation and disparity cease to be important for depth perception. The degree of accommodation required by the muscles controlling the lens appears to have some importance, but other perceptual factors also contribute. Constancy of colour in the face of lighting changes and size, depth perception and most other percepts result from an individual's ability to continually synthesise past experience and current sensory cues. D R Campbell School of Computing University of Paisley 29 15